Saimaa University of Applied Sciences

Technology, Imatra

Degree Programme in Paper Technology

Specialization in Paper Technology

Yang Aohan

COATING, TITANIUM DIOXIDE AND SOLAR

CELL

Bachelor’s Thesis 2011

ABSTRACT

Yang Aohan

Coating, Titanium Dioxide and Solar Cell, 48 pages

Saimaa University of Applied Sciences, Imatra

Technology, Degree Programme in Paper Technology

Bachelor’s Thesis, 2011

Tutor: Pasi Rajala, Principal lecturer, SUAS

The objective of this bachelor’s thesis is to get basic ideas about coating and a

deep understanding of properties of titanium dioxide pigments as well as their

application and performance in solar electricity energy technology. This thesis

consists of three main parts, eight chapters.

The first part is about basic knowledge of coating and tests of coated paper.

Coating pigments are generally introduced in the part.

In the second part, coating additives are introduced in details from two aspects –

optical brightening agents and titanium dioxide. Especially properties of titanium

dioxide are introduced here.

The last part is about the solar cell. The key point for this part is about the new

type of solar cell which is based on dye-sensitized nanocrystalline titanium

dioxide.

Keywords: Coating, Optical Brightening Agents, Titanium Dioxide, Coating

Pigments, Smart Material, Dye-sensitized Solar Cell, Titanium Dioxide Dyed

Solar Cell

CONTENTS

LIST OF SYMBOLS ............................................................................................ 5

1 INTRODUCTION ............................................................................................. 6

2 PIGMENT COATING ....................................................................................... 7

2.1 Objective of Coating .................................................................................. 7

2.2 Coating Pigments ...................................................................................... 7

2.2.1 Functions of Coating Pigments ............................................................ 7

2.2.2 Ideal Pigment ...................................................................................... 7

2.2.3 Pigment Properties .............................................................................. 8

2.2.4 Classification of Pigments ................................................................... 9

2.3 Testing of Pigment Coated Papers ............................................................ 9

2.3.1 Brightness ........................................................................................... 9

2.3.2 Opacity .............................................................................................. 11

2.3.3 Smoothness ...................................................................................... 12

2.3.4 Gloss ................................................................................................. 13

2.3.5 Ink Receptivity ................................................................................... 13

2.3.6 Surface Strength ............................................................................... 14

3 COATING ADDITIVES ................................................................................... 15

3.1 Optical Brightening Agents ...................................................................... 15

3.1.1 Fluorescence and Chemical Structure............................................... 15

3.1.2 Types of OBA .................................................................................... 17

3.1.3 The Role of Carrier ............................................................................ 17

3.2 Titanium Dioxide ...................................................................................... 18

3.2.1 Introduction of Titanium Dioxide ........................................................ 18

3.2.2 Properties of Titanium Dioxide Pigments ........................................... 18

3.2.3 Opacity .............................................................................................. 20

3.2.4 Colour ................................................................................................ 22

3.2.5 Gloss ................................................................................................. 22

3.2.6 Dispersibility ...................................................................................... 23

3.2.7 Electrical Properties........................................................................... 24

4 APPLICATIONS ............................................................................................. 26

4.1 Applications of Titanium Dioxide .............................................................. 26

4.1.1 Paper ................................................................................................. 26

4.1.2 Plastic and Rubber ............................................................................ 27

4.1.3 Solar Cell ........................................................................................... 28

5 SOLAR CELL ................................................................................................. 29

5.1 Introduction .............................................................................................. 29

5.1.1 What is Solar Cell .............................................................................. 29

5.1.2 Cost of Solar Electricity ..................................................................... 30

5.2 Different Solar Cells ................................................................................. 31

5.2.1 Single-crystal and Polycrystalline Silicon Solar Cell .......................... 31

5.2.2 Thin Film Solar Cell ........................................................................... 32

5.2.3 III-V Semiconductors ......................................................................... 33

5.2.4 Photoelectrochemical Solar Cells ...................................................... 33

5.3 Materials of Solar Cells ............................................................................ 34

5.4 Titanium Dioxide Solar Cell ..................................................................... 34

6 WORKING PRINCIPLE OF TiO2 SOLAR CELL ............................................ 35

6.1 Theoretical Issue of TiO2 Dye Cell Operation .......................................... 36

6.1.1 Light Absorption ................................................................................ 37

6.1.2 Charge Separation ............................................................................ 37

6.1.3 Charge collection ............................................................................... 37

7 PERFORMANCE AND APPLICATION OF TiO2 DYED SOLAR CELL .......... 41

7.1 Energy Conversion Efficiency .................................................................. 41

7.2 Long Term Stability .................................................................................. 42

8 SUMMARY..................................................................................................... 44

SOURCES ........................................................................................................ 46

LIST OF SYMBOLS

OBA – optical brightening agents

UV – ultraviolet

ISO – International Organization for Standards

CIE – the International Commission on Illumination

TiO2 – titanium dioxide

PV – photovoltaic

CWSN – critical wax strength number

a-Si – amorphous silicon

CdTe – cadmium telluride

CIGS – copper indium gallium diselenide

DSSC – dye-sensitized solar cell

a-Si – amorphous silicon

CIGS – copper indium gallium diselenide

℮- – electron

γ – the value of paper measured under conditions C/2°

γ0 – the the γ value of a single sheet measured under the same conditions

(with same geometry) against a black backing

F – the reflectivity of the coating

– the reflectivity of the coating

– the refractive index of the binder in which the pigment is incorporated

6

1 INTRODUCTION

Pigment coating provides paper and paperboard enhanced smoothness, better

ink receptivity, more whiteness, and more gloss (Kocurek 1990, 4). As the most

important white pigment currently produced commercially and the most

expensive coating pigments used in paper industry, titanium dioxide pigments

have many requirements which an ideal pigment needs (Gullichsen 2000).

For balancing the mismatch of supply and demand of energy, sustainable and

renewable generation of energy/electricity by photovoltaic systems has been

developed. Storage of energy is often essential for these systems to become a

competing technology for the conventional energy generation. One way to solve

the problem of storage is the development of photochemical systems that use

sunlight (photons) directly to drive reversible chemical reactions leading to

products that can be used for energy storage. (Sommeling, et al., 2008., Halme

2002)

Titanium dioxide is not widely used in paper industry only, because of its flexible

manufacturing process, low manufacturing cost, high efficiency, low

manufacturing cost and long term stability etc., it is also in a growing number of

applications in solar cell. A new type of solar cell which is based on

dye-sensitized nanocrystalline titanium dioxide has been developed.

(Sommeling & Späth 2008.)

7

2 PIGMENT COATING

2.1 Objective of Coating

Coating improves the printing properties of paper; it provides the printer with a

sheet with a superior surface for printing (Kocurek 1990, 4). Pigment coating

provides enhanced smoothness, better ink receptivity, more whiteness, and

more gloss. Thus, quality printed papers are boards are coated. (Gullichsen

2000, 14.) Coating does not cover imperfections in the underlying paper, but it

makes them more noticeable. (Kocurek 1990, 12).

2.2 Coating Pigments

2.2.1 Functions of Coating Pigments

Pigment is the most abundant component in coating. Thus, pigment is

naturally the most essential factor affecting the properties of coating. And the

volume fraction is important. The smaller the particle size is, the glossier will

be the coating. (Gullichsen 2000, 14-17.)

2.2.2 Ideal Pigment

Every particle must be clusters separated and wetted for obtaining the

maximum benefit from any pigment. When choosing a certain type or types of

coating pigment, the smaller the particle size is, the glossier will be the coating.

However, not only will large aggregates make the coating rough, in some

special cases, they will break down under supercalendering and cause dusting.

(Lewis 1988, 1-8.)

8

2.2.3 Pigment Properties

If an ideal pigment existed, it would have the following properties:

- Good chemical stability and low solubility in water

- Good brightness (Good light reflectivity)

- Free from impurities

- Appropriate particle size and proper particle size distribution

- Good opacity (High refractive index)

- Small binder demand

- Good flow properties as an aqueous suspension

- Good dispersability

- Good glossing properties

- Outstanding compatibility with other coating components

- Low density

- Non-abrasive

- Low water absorption

- Cheap

Of course different pigments have different combinations of these properties,

there is no pigment could have all. Titanium dioxide has many of them.

(Gullichsen 2000, 61.)

9

2.2.4 Classification of Pigments

Pigments could be classified in lots of ways. A commonly used way is to

classify pigments in the following way:

- Main pigments

- Special pigments

- Additional pigments

Main pigments are those pigments which form the major fraction of the

pigment part of a coating colour. Special pigments are similar to main

pigments except their limited applications. Pigments that form the minor

fraction of the pigment part of a coating colour belong in the class of additional

pigments. Titanium dioxide is a commonly used additional pigment.

(Gullichsen 2000, 62.)

2.3 Testing of Pigment Coated Papers

The coated surface of paper must be of such quality that it is optically acceptable,

ink-receptive and durable for the carrier for images that will be received during

printing. Thus, brightness, opacity, smoothness, gloss, ink receptivity, and

surface strength are important for pigment coated paper. (Kocurek 1990, 15.)

2.3.1 Brightness

In pigment coated paper, the coating paper grade structure is based primarily on

brightness. Because the brightness of pigment coated paper depends heavily on

the brightness of raw stock, the raw stock should have a brightness which is very

close to that of the dry coating in practice. (Kocurek 1990, 15.)

10

Brightness is an intrinsic reflectance factor, which is determined with a

brightness meter whose sensitivity to light agrees with ISO standard 2470.

(Gullichsen 1999, 173.)

Figure 2.1 Sensitivity distribution of ISO brightness and Y-value (Gullichsen

1999)

Figure 2.1 shows the sensitivity spectrum. The peak of the sensitivity spectrum

corresponds to the wavelength of 457 nm. This means the origin of the

abbreviation R457. (Gullichsen 1999, 173.)

There are two standards for brightness from TAPPI: T 525 – Pulp (diffuse

illumination and 0°observation) is identical to ISO 2470. And T 452 – Pulp, paper

and paperboard (directional reflectance at 457 nm) is identical to different

geometry. (Gullichsen 1999, 173, 174.)

The American paper industry uses the standard brightness instrument which has

45°illumination and 0°viewing. According to the test method TAPPI T 452, the

11

measurement is through a 457 nm filter and is based on the reflectance of

magnesium oxide as being 100%. (Kocurek 1990, 15.)

2.3.2 Opacity

Opacity is defined as the measure of a material’s ability to obstruct the passage

of light. When no light is transmitted the opacity is 100%. (Kocurek 1990, 15.)

There are two commonly used methods to determine opacity:

Opacity, γ 0 / γ) 100 (1)

Where:

γ is the value of paper measured under conditions C/2°.

γ0 is the γ value of a single sheet measured under the same conditions

(with same geometry) against a black backing.

The determination is described by standard methods ISO 2471, SCAN-P 8 and

TAPPI T 519. (Gullichsen 1999,175.)

In practice, the opacity is usually given by a value which is known as the contrast

ratio, which is the ratio of the diffuse reflectance from a same sheet of paper

backed by a black body to the reflectance backed by a white body but the same

area. The contrast ratio is an indirect measurement which measures the amount

of transmitted light. (Kocurek 1990, 15)

12

Contrast Ratio = Opacity ( 89% reflectance backing ), % = ( R0 / R0.89 ) 100 (2)

Where:

R0 is the reflectance factor measured against a black backing under standard

conditions.

R0.89 is the reflectance factor of the same sheet measured against a backing with

a reflectance of 89%.

There is difference between these two opacity determination methods. The

devices which are used for measurements represent different measurement

geometries and illuminants. Thus, the results are not comparable. (Gullichsen

1999, 175.)

2.3.3 Smoothness

The principle reason for pigment coating is to produce paper a smooth surface

which can ensure good contact between the coated paper and the printing plate.

An irregular or rough surface with many discontinuities cannot receive a

satisfying impression. (Kocurek 1990, 15.) The purpose of smoothness

measurement is to find a figure describing the topography of the paper surface

simply (Gullichsen 1999, 156).

Smoothness is determined by the rate of airflow between a sheet of paper and a

plane surface in contact with the paper. The rate of flow between the two

surfaces is proportional to the volume of air voids.

TAPPI Method T 479 is used to test of smoothness with the Bekk instrument.

Bendtsen and Sheffield smoothness tests are used with TAPPI Useful Methods

535 and 518, respectively. (Gullichsen 1999, 15.)

13

2.3.4 Gloss

Gloss is the attribute of a surface which makes the object look shiny or lustrous,

and it is a measure of the degree to which a coated surface approaches a

perfect specular surface or mirror in its ability to reflect light (Gullichsen 1999,

16). Gloss is affected by specular reflection from the surfaces. Gloss values of

pigment coated paper fall with broad range. High gloss is desired in a high

quality, prestigious printed image. Gullichsen 1999, 164.)

There are different gloss measurement methods for measuring different aspects

of the phenomenon. The gloss of papers and paperboards vary widely, so two

measurement methods are necessary. TAPPI Methods T 480 and T 653 are

used in gloss tests. (Gullichsen 1999, 164.)

The specular angle refers to incident and viewing angles measured from the

perpendicular plane of the surface; it is selected by experiments that showed the

best correlation with visual ranking at this angle. TAPPI T 480 uses a 75°angle,

and TAPPI T 653 uses a 20°angle. TAPPI T 653 is used for evaluation of

high-gloss papers and boards such as lacquered papers and cast-coated papers.

(Gullichsen 1999, 164)

2.3.5 Ink Receptivity

Printing ink acceptance of paper will be improved significantly when pigment

coating is applied to raw stock.

TAPPI Useful Method 553 is used to test ink receptivity. K&N test from TAPPI

Useful Method 553 is applied, to the coated surface is used a special ink

containing an oil-soluble dye dissolved in a nondrying varnish, which allows the

ink to penetrate for a set time and the excess is the wiped off.

14

The reflectance of the ink surface measures the ink receptivity. The higher the

ink receptivity is, the lower the reflectance value will be. (Gullichsen 2000, 16.)

2.3.6 Surface Strength

Surface strength is defined as the ability of the paper to resist a force pulling out

fibers or fiber bundles from its surface. A low surface strength may make linting

problems and destroy the printing result or cause runnability problems.

(Gullichsen 1999, 148.)

Methods for surface strength testing of paper usually use high viscosity, tacky

oils or printing inks. A commonly used tester is the one developed by IGT and

described in ISO 3738. This unit prints the paper using very tacky oil, at a

continuously rising speed. The printing speed at the picking of the surface

begins is measured. The viscosity of the oil and the product of the speed is a

measure of the surface strength of the paper. (Gullichsen 1999, 149, 150)

Another test method is the Dennison wax pick test, TAPPI T 459. In this test,

sealing wax sticks with different picking strength values are fastened on paper

by melting. The wax number, at which the papaer surface does not yet break

when removing the wax piece, indicates the surface strength. The critical was

strength number (CWSN), which is the numerical surface strength value.

(Gullichsen 1999, 149, 150.)

The resistance, with the coating is to be picked or pulled off the raw stock during

printing, is determined by pick strength. The printing quality will be poor when

picking of the coating surface occurs. Pick strength is determined by the amount

of binder present. (Gullichsen 2000, 16.)

15

3 COATING ADDITIVES

3.1 Optical Brightening Agents

Additional pigments improve printing properties. The application of additional

pigments leads to improvement of optical properties, gloss, and ink absorption.

The brightness of paper and board has grown stunning during the recent years.

The brightness of pulp, fillers and coating pigments is not high enough to reach

the brightness targets. Thus, there is a need to use optical brightening agents

(OBA). (Gullichsen 2000, 298.)

3.1.1 Fluorescence and Chemical Structure

“Fluorescence is a phenomenon where the molecules of a fluorescent substance

become electronically exited by absorbing light energy and then emit this energy

at a higher wavelength. Fluorescence is usually restricted to compounds with

large conjugated systems containing -electrons. Most of the OBAs on the

market are derivatives of bis (triazinylamino) stilbene.”( Gullichsen 2000, 298.)

Figure 3.1 Structural formula of bis (triazinylamino) stilbene (Gullichsen 2000,

299)

16

The strong fluorescence is only exhibited in the trans-isomer. Properties like

substantivity and solubility are influenced by the nature of X and Y.

The OBAs used in paper industry are water-soluble because they are Na-salts.

OBAs absorb ultraviolet radiant energy at 300-360 nm and re-emit the energy in

the visible range mainly in the blue wavelength region. This results in higher

brightness or whiteness by increasing the amount of light emitted. And bluish

reflected light results in the compensation of yellow shade of paper, contributing

to making whiter look of paper. (Gullichsen 2000, 299.).

Figure 3.2 Functioning mechanism of OBA in paper coatings (Gullichsen 2000,

299)

Whiteness is the degree to which a surface is white; it is defined as the

reflectance of light across the visible spectrum including colour components in

the measurement. Brightness again is an attribute of visual perception in which a

source appears to be radiating or reflecting light. It is defined as the relectance of

light at the wavelength of 457 nm without colour in the measurement.

(Gullichsen 2000, 299.)

17

Instead of the traditional ISO brightness which does not define the illuminant for

use in the determination, to measure the brightness or whiteness of paper and

board containing OBAs, the test method used more and more is the CIE

whiteness, which requires an instrument having a known amount of UV in the

illumination. (Gullichsen 2000, 299.)

3.1.2 Types of OBA

Based on the stilbene molecule described in Fig 3.1, there are differences

between the number of solubilizing sulfonic groups. The differences of the

optical brighters are divided to disulfonated OBAs, tetrasulfonated OBAs and

hexasulfonated OBAs. (Gullichsen 2000, 299.)

Disulfonated OBAs have two sulfonic groups and very good affinities, they are

mostly used in the wet end. But this OBA has limited solubility.

Tetrasulfonated OBAs are the most commonly used. They have medium affinity

and good solubility. They can be mostly applicated in wet end, size-press and

coating in paper industry.

Hexasulfonated OBAs can be added up to 15% of pigment amount since they

are very soluble. They are specialties used in coatings where high brightness is

required. (Gullichsen 2000, 295.)

3.1.3 The Role of Carrier

A good carrier needs to be linear and contain hydroxyl or other hydrophilic

groups. OBAs work only when they are fixed to a carrier. (Gullichsen 2000, 300.)

18

The contact between the carrier and OBA is increased by linearity, so physical

bonds can be formed between the OBA and hydrophilic groups of the carrier.

(Gullichsen 2000, 300.)

UV exposure causes yellowing of brightened paper, but a good carrier increases

the light stability, so the yellowing of paper might be compensated. Futhermore

the carrier influence the migration of OBA; it works better in against the migration

when the carrier is more efficient. (Gullichsen 2000, 299.)

3.2 Titanium Dioxide

3.2.1 Introduction of Titanium Dioxide

Titanium dioxide is the ninth most abundant element in the earth’s crust, it is the

most important white pigment currently produced commercially (Gullichsen 2000,

121).

“Pure titanium dioxide (TiO2) is a colourless crystalline solid. As with other

d-block elements in its group of the Periodic Table, TiO2 is stable, nonvolatile,

largely insoluble, and rendered refractory by strong ignition. ” (Lewis 1988, 17.)

TiO2 pigments are one of the most expensive pigments used in paper industry

(Gullichsen 2000, 121). They are synthetic pigments produced by expensive raw

materials and complicated process (Lewis 1988, 1).

3.2.2 Properties of Titanium Dioxide Pigments

Titanium dioxide pigments have extremely high refractive indices, whiteness,

and high reflectance in the visible region of light and optimal particle size. These

19

properties make them the most efficient white pigments.

Titanium dioxide is the most stable white pigment; due to its stability, titanium

dioxide is non-toxic and considered to be a very safe material.

There are two forms of titanium dioxide pigments: rutile and anantase.

(Gullichsen 2000, 125.)

Figure 3.3 Crystal structure of anatase titanium dioxide.

(http://cst-www.nrl.navy.mil/users/sullivan/TiO2/tio2.html)

20

Figure 3.4 Crystal structure of rutile titanium dioxide.

(http://cst-www.nrl.navy.mil/users/sullivan/TiO2/tio2.html)

Anatase pigment has a less compact crystal structure than rutile pigment, which

is the reason for its lower refractive index, less stability and lower density.

(Gullichsen 2000, 125.)

3.2.3 Opacity

The most essential property of white pigment is its ability to opacify and whiten

the medium in which it is dispersed. The opacifying potential is controlled by two

properties ---- refractive index and particle size. Rutile titanium dioxide has the

highest refractive index of all the available white pigments. This property gives

titanium dioxide the greatest possible opacifying potential. (Lewis 1988, 15-18.)

Refractive index is not a fixed property but varies with the wavelength of light, as

illustrated in Figure 3.5:

21

Figure 3.5 Refractive indices of synthetic rutile crystal and natural anatase

crystal as a function of wavelength (Lewis 1988, 17)

An indication of the influence of refractive index on opacity can be achieved by

Fresnel equation:

(3)

Where F is the reflectivity of the coating, is the refractive index of the

pigment, is the refractive index of the binder in which the pigment is

incorporated. By this equation, an indication of the potential opacity of a series of

pigments can be calculated. (Lewis 1988, 17.)

22

3.2.4 Colour

Two aspects of colour are important to the users of titanium dioxide

pigments---whiteness and undertone.

Whiteness is the impression produced when the principal colouring matter is the

white pigment itself; it is enhanced by careful purification of the pigment during

the manufacturing process, and it is particularly important to reduce the

concentration of contaminant elements with coloured oxides to a minimum.

(Lewis 1988, 20.)

Undertone is defined as the colour obtained when such a pigment is greatly

extended with white, it is usually applied to coloured pigments. The undertone of

a white pigment is the hue of a gray paint containing the pigment in admixture

with a standard black pigment. Undertone is dependent on the particle size of

the titanium dioxide mostly, thus it can be controlled by manufacturer. (Lewis

1988, 20.)

3.2.5 Gloss

According to Lewis (1988, 21) gloss is a sensation experienced by an observer

when light is reflected from a surface; it is an essential parameter in surface

coating. Gloss is influenced by the distribution and concentration of titanium

dioxide pigments in the paint film. When the paint film is optically smooth, its

specular gloss will be directly related to the refractive index of the film surface;

however, paint films generally have a degree of surface roughness. The surface

roughness can be considered to consist of two components-macro roughness

and micro roughness. The result for macro roughness of surface defects is of the

order of 1 µm or more in height; micro roughness is attributable to defects of the

order of 0.6 µm or less. Thus, it is primarily micro roughness which is affected by

23

the presence of titanium dioxide pigment particles at or near the surface of the

paint. (Lewis 1988, 21.)

The primary reason for incorporating titanium dioxide pigment in paint films is

producing opacity. However, increasing the level of pigmentation to achieve high

opacity also causes decrease of the observed gloss. In the case of latex paints,

where no such clear layer exists, the initial observed gloss values are lower and

tend to fall as pigmentation rise, although it is predicted that increasing the

refractive index of a paint film will increase the observed gloss value theoretically.

The reason is, as the pigment loading increases, the presence of pigment

particles near the surface of the film causes a marked increase in surface micro

roughness, thus, observed gloss decreases. (Lewis 1988, 22.)

3.2.6 Dispersibility

To obtain the maximum benefit from any pigment, every particle must be wetted

and clusters of particles separated. Because large aggregates make the coating

rough and cannot be bonded properly to the base stock; in extreme cases, they

will break down under supercalendering and cause dusting. (Kocurek 1990, 18.)

In relative terms, titanium dioxide pigments are easily dispersible. As a result of

surface treatment, titanium dioxide can be processed using a wide variety of

modern machinery and provide correct mill-base formulating procedures have

been followed. (Lewis 1988, 24, 25.)

There are agglomerates which need energy to be dispersed in water in dried

titanium dioxide pigments. Several types of dispersion equipment can be used

successfully to prepare titanium dioxide dispersions. (Lewis 1988, 24-27.)

Commonly, titanium dioxide pigments are dispersed by various amounts of

24

sodium polyacrylate. Instead of sodium polyacrylate, other dispersing agents

such as special polyacrylate (mixed salt of an acrylic polymer) or amino alcohols

should be used, for dispersing uncoated rutile and anatase. (Gullichsen 2000,

129.)

3.2.7 Electrical Properties

Pure titanium dioxide is a semiconductor, which means its electrical conductivity

is very low (approximately 10-10 – 10-13 S/cm). Thus, pure titanium dioxide can be

considered as an insulator. However, if conductivity increases by a factor of 107

over the temperature range 30-420°C, conductivity will also rise if reduction

occurs to produce Ti3+ ions within the Ti4+ crystal lattice.

Details of the electrical properties of titanium dioxide are show in table 3.1,

including average dielectric constants valid for polycrystalline solids, which the

crystallites are freely oriented. (Lewis 1988, 26, 27.)

25

Table 3.1 Selected electrical properties of titanium dioxide (Lewis 1988)

Since the high dielectric constant of titanium dioxide and other titanium

compounds, the electrical properties of rutile titanium dioxide are important in

certain plastics and ceramics applications, to advantage in the manufacture of

electrical condensers and other electronic components. (Gullichsen 2000, 128.)

26

4 APPLICATIONS

4.1 Applications of Titanium Dioxide

Titanium dioxide has a wide range of applications. Although a general indication

of titanium dioxide pigments has already been given, each individual consumer

industry has specific requirements. (Lewis 1988, 32.)

4.1.1 Paper

In paper industry, about 14% of titanium dioxide pigments are used in paper

industry as filler or coating pigments (Gullichsen 2000, 128). Titanium dioxide

pigments are used as a direct beater addition to pigment the paper stock and in

the manufacture of coatings (Lewis 1988, 39).

The main function of titanium dioxide in paper applications is to increase the

opacity and brightness of the final product, but it will have an effect on the

mechanical properties of the sheet. Parameters such as burst and tensile

strength are affected by filler content; (Lewis 1988, 39) which means increasing

the amount of titanium dioxide pigment in a paper will cause a progressive

decrease in strength. However, as titanium dioxide is always used in conjunction

with big amounts of clay, its influence on the mechanical properties of sheet can

be ignored. (Gullichsen 2000, 128.)

For direct beater addition, uncoated anatase pigments are widely used; because

uncoated anatase pigments are self-dispersing in water and cost less than their

rutile equivalents. (Lewis 1988, 39.)

Both anatase and rutile pigments are used in the manufacture of paper coatings,

27

which are delivered as a high- solids (> 65%) slurry. Application of a coating to

the paper surface improves the printing properties of an otherwise variable

substrate. When optical brighteners are used in the paper coating, anatase

grades must be employed because of the high UV absorption characteristics of

rutile pigments. (Lewis 1988, 39.; Gullichsen 2000, 128.)

In paper industry, a highly specialized application is the manufacture of paper

laminates. For such systems, the main requirement of pigments is a high

resistance to discolouration, which is caused by partial reduction of titanium to its

trivalent state, in the presence of UV radiation and the melamine resin used to

impregnate the paper. Rutile pigments are preferred for such applications

generally, but they require a secondary heat treatment or a highly specialized

surface treatment to give a satisfactory level of performance. (Lewis 1988, 39.)

4.1.2 Plastic and Rubber

Titanium dioxide pigments are principally used in plastics to impart whiteness

and opacity. Dispersibility is an important pigmentary property in many plastic

systems. Satisfactory levels of dispersion can more readily be achieved by

employing siliconetreated pigments for certain application areas. (Lewis 1988,

38.)

Today, titanium dioxide plastic is a new photovoltaic material. The driving force

for the development of researching for new photovoltaic materials for solar cell is

the reduction of the cost of photovoltaic cells and modules. (Halme, 2002.)

28

4.1.3 Solar Cell

As the properties of good light absorption and stability, titanium dioxide has been

developed in making solar cell that is based on dye-sensitized nanocrystallin

titanium dixide. Chapters 5.4 and 6 will introduce applications of titanium dioxide

pigments in solar cell industry in detail.

29

5 SOLAR CELL

5.1 Introduction

Sustainable and renewable generation of energy/electricity by photovoltaic

systems is being developing because the mismatch of supply and demand of

energy. Storage of energy is often essential for these systems to become a

competing technology for the conventional energy generation. One way to solve

the problem of storage is the development of photochemical systems that use

sunlight (photons) directly to drive reversible chemical reactions leading to

products that can be used for energy storage. (de Boer 2011)

Solar electricity energy technology is growing steadily nowadays. Producing

electricity directly from solar radiation is near to an ideal way to use natural

renewable energy. Developing more economical material for making solar cell is

an essential topic today.

5.1.1 What is Solar Cell

Solar cell is also called photovoltaic cell or photoelectric cell; it is a solid state

device which converts the energy of sunlight directly into electricity by the

photovoltaic effect. (http://en.wikipedia.org/wiki/Solar_cell)

Some cells are designed to efficiently convert wavelengths of solar light which

reach the Earth surface. However, other solar cells are optimized for light

absorption beyond Earth’s atmosphere. (Sommeling, et al., 2008.)

The oldest photovoltaic cell is the photoelectrochemical solar cell, for this solar

cell a semidonductor-electrolyte junction is used as a photoactive layer. Energy

30

conversion efficiencies exceeding 16% have been achieved with the

photoelectrochemical solar cells utilizing semiconductor photoelectrodes

(Meissner 1999), these solar cells have been left without practical importance

because of instability by photocorrosion. (Halme 2002).

A new type of solar cell which is based on dye-sensitized nanocrystalline

titanium dioxide has been developed by M. Grätzel and coworkers (O’ Regan

and Grätzel 1991; Nazeeruddin, et al. 1993). Remarkably high quantum

efficiencies have been reported for this type of solar cell (also called nc-DSC),

with overall conversion efficiencies up to 11 %. (Green, et al. 1998.)

5.1.2 Cost of Solar Electricity

Power generation by grid connected PV systems might be the main application

today, and it is pursued to be realized in the future in a large scale. Thus, the

cost of PV system needs to be reduced significantly; which means the cost of

solar electricity needs to be reduced. (Halme 2002.)

There are two ways to reduce the cost of PV system: improvement of

performance (efficiency) and reduction of direct manufacturing cost. With this

respect, two requirements are needed: high efficiency and low manufacturing

cost. (Halme 2002.)

31

5.2 Different Solar Cells

There are different solar cells. They are single-crystal and polycrystalline silicon

solar cell; Thin film solar cell; III-V Semiconductors and Photoelectrochemical

solar cells.

5.2.1 Single-crystal and Polycrystalline Silicon Solar Cell

The first silicon solar cell was developed by Chapin, Fuller and Pearson at the

Bell Telephone Laboratories in the mid 1950's. There was already about 6%

efficiency for the first silicon solar cell (Kazmerski 1997). The silicon solar cells

occupy 82% of the photovoltaic market today and the record efficiency for a

laboratory cell is 24.7% (Green 2001). The efficiency of the commercial

crystalline silicon solar panels is the best which occupies about 15% (Shah et al.

1999).

Because of the high quality Si has been produced at large quantities by the

semiconductor industry, Si dominates the PV market. The processing of

crystalline silicon wafers is high-level semiconductor technology, and it is very

capital intensive and expensive. And this cost is added directly to the cost of the

photovoltaic modules. So the cost of processed silicon wafers contribute to fifty

per cent of the total manufacturing cost of the module. (Goetzberger & Hebling

2000.)

There is a big question for the Si photovoltaic technology - the availability of

highly purified Si. The PV industry has been using mainly low cost reject material

from the semiconductor industry. This gives a problematic dependence on the

volatile semiconductor market which causes fluctuations and also the problem of

the cost of Si material for the solar cells. It is generally seen that the dominance

of the standard Si PV technology in the growing PV market can be realized only

32

by production of special solar cell grade silicon actually. However, because of

the high purity requirements and the small market for the special silicon, the first

efforts to produce such material have been unsuccessful at the moment.

(Goetzberger & Hebling 2000.)

While the crystalline silicon technology is relatively mature, there is still a large

potential of cost reduction. However, the cost reduction can be achieved only by

increasing manufacturing volume. In manufacturers' opinion, this is coupled to

the need for the special solar cell grade silicon supply and from the market's

point of view not very easy to achieve at 20 percent PV system costs without

governmental subsidies for customers to create artificial markets for the solar

electricity. (Goetzberger & Hebling 2001.)

5.2.2 Thin Film Solar Cell

The crystalline silicon is normally referred to as the first generation photovoltaic

technology. The second generation photovoltaic consists of thin film solar cell

materials such as cadmium telluride (CdTe), amorphous silicon (a-Si), copper

indium gallium diselenide (CIGS), and thin film crystalline silicon. (Green 2001.)

The thin film solar cells have been developed because of their potential for the

reduction of manufacturing costs. While silicon solar panels are assembled from

individual cells process starts from about 100 cm2 silicon wafers, while thin film

semiconductor materials can be deposited onto large surfaces, which is

beneficial for volume production. (Green 2000b.)

The thin film semiconductor materials have much higher absorption coefficient

than silicon also, as the direct band gap semiconductors, and therefore typically

less than 1 mm thick semiconductor layer is required, which is 100-1000 times

33

less than for Si. Thus, the cost of semiconductor material is reduced, more

expensive semiconductors can be used in the thin films. (Goetzberger & Hebling

2000.)

5.2.3 III-V Semiconductors

Semiconductors such as GaAs, GaAlAs, GaInAsP, InAs, InSb, and InP are

interesting solar cell materials. They have near-optimal band gaps and are

extremely expensive. These materials have been only found applications in the

space solar cells. So the performance of III-V Semiconductors is more important

criterion than their costs. And to some extent in concentrating systems where the

active surface area of the cells can be reduced significantly, thus, expensive

materials can be used. (Halme 2002.)

5.2.4 Photoelectrochemical Solar Cells

Photoelectrochemical solar cell is the oldest type of photovoltaic cell, which was

used already by Becquerel for the discovery of the photovoltaic effect in 1839. A

semiconductor-electrolyte junction is used as a photoactive layer in the

photoelectrochemical solar cell. While energy conversion efficiencies exceeding

16% have been achieved with the photoelectrochemical solar cells utilizing

semiconductor photoelectrodes, instability by photocorrosion makes them have

no practical importance. Furthermore, the photoelectrochemical solar cells using

same semiconductor materials as in the commercial solar cells, such as Si,

CuInSe2 or GaAs do not offer any real advantages over the established solid

state solar cells. (Meissner 1999.)

34

5.3 Materials of Solar Cells

Materials for efficient solar cells must have characteristics match to the spectrum

of available light. Light absorbing materials can often be used in multiple

physical configurations to take advantage of different light absorption and charge

separation mechanisms. (Sommeling, et al., 2001, 1.)

5.4 Titanium Dioxide Solar Cell

As Sommeling and Späth (2008) point out titanium dioxide solar cell has the

potential to reach low costs in future outdoor power applications; it is the solar

cell which is based on dye-sensitized nanocrystalline titanium dioxide.

The application of titanium dioxide solar cell requires only a low power output,

since this is easier to achieve. Less stringent efficiency requirements of titanium

dioxide solar cell make the manufacturing process more flexible, i.e. the

requirements for materials used are less severe.

Lower manufacturing costs and more flexibility in manufacturing process open

the way for alternative production processes. (Sommeling & Späth 2008.)

35

6 WORKING PRINCIPLE OF TiO2 SOLAR CELL

Titanium dioxide, which is sensitized for visible light by a monolayer of adsorbed

dye, works as a bandgap semiconductor. The most frequently used dye is, for

reasons of best efficiencies,

cis-(NCS)2bis(4,4’-dicarboxy-2,2’bipyridine)-ruthenium(II). The photoelectrode in

such a device consists of a nanoporous TiO2 film (approx. 10μm thick) deposited

on a layer of ransparent conducting oxide (TCO, usually SnO2:F) on glass or

plastic, as can be seen in figure 6.1.

Figure 6.1 Working Principle of TiO2 Solar Cell (Sommeling & Späth 2008)

Photo and counter electrode are clamped together in a complete cell, and the

space between the electrodes and the voids between the titanium dioxide

particles are filled with an electrolyte which consists of an organic solvent

containing a redox couple; the electrolyte is usually iodide /triiodide (I-/I3-).

(Sommeling & Späth 2008.)

36

Less than 1 % of the incoming light (one sun conditions) is absorbed by a dye

monolayer on a flat surface. To obtain reasonable efficiencies comparable to

established solar cell technologies, the surface area is enlarged by a factor of

1000, by using nanoparticles of TiO2 with a diameter of approximately 10-20 nm.

(Sommeling & Späth 2008.)

The working principle of the titanium dioxide solar cell is based on excitation of

the dye followed by fast electron injection into the conduction band of the TiO2,

leaving an oxidized dye molecule on the TiO2 surface. Injected electrons

percolate through the TiO2, and then they are fed into the external circuit. At the

counter electrode, triiodide is reduced to iodide by metallic platinum under

uptake of electrons from the external circuit:

I3- + 2e- → 3I (4)

Iodide is transported through the electrolyte towards the photoelectrode, where it

reduces the oxidized dye. After this process, the dye molecule is ready for the

next excitation/oxidation/reduction cycle. (Sommeling & Späth 2008.)

6.1 Theoretical Issue of TiO2 Dye Cell Operation

The operation of a photovoltaic cell can be generally divided into three basic

steps:

Light absorption

Charge separation

Charge collection

37

6.1.1 Light Absorption

While the high efficiency of the dye sensitized solar cell starts from a collective

effect of numerous well-tuned physical-chemical nanoscale properties as later it

will become apparent, the key point is the principle of dye-sensitization of large

band-cap semiconductor electrodes. In the DSSC, sensitization dyeing is made

by coating the internal surfaces of the porous TiO2 electrode with special dye

molecules tuned to absorb the incoming photons. (Hagfeldt & Grätzel 2000.)

6.1.2 Charge Separation

The charge separation in DSSCs is based on an electron transfer process from

the dye molecule to TiO2 and a hole transport process from the thereby oxidized

dye to the electrolyte. The electron transfer mechanism is affected by the

electronic structure of the adsorbed dye molecule and the energy level between

the excited state of the dye and the titanium dioxide conduction band. (Hagfeldt

& Grätzel 1995.)

Furthermore, there are no significant macroscopic electric fields present

between the individual nanoparticles in vast majority of the electrode. In this

case, lack of band bending is a result of the individuality of all the nanocrystalline

particles: If the film is ensemble, the sufficiently thick nanoparticle film could

have a collective space charge. However, the electrolyte surrounding all the

particles effectively decouples the particles and screens any existing electric

fields to about a nanometer maximum. (Pichot & Gregg 2000b.)

6.1.3 Charge collection

In the DSSCs charge transport happens by electron transport in the

nanostructured TiO2 electrode and hole transport in the electrolyte as I3-. Both

38

charge transport mechanisms are equally important for the operation of the solar

cell. (Hagfeldt & Grätzel 2000.)

Ion transport in the redox electrolyte

Usually the electrolyte in the DSSCs is an organic solvent which contains the

redoxes pair I-/I3-. In this case, it works as a hole-conducting medium. At the TiO2

electrode the oxidized dye, left behind by the electron injected to the TiO2, is

regenerated by I- in the electrolyte in the reaction:

2S+ + 3I- → 2S + I3- (5)

While, oppositely, electrode I3- is reduced to I- in the reaction:

I3- + 2℮- (Pt) → 3I- (6)

I3- is produced at the TiO2 electrode and consumed at the counter-electrode,

thus diffused across the electrolyte correspondingly. Because of this, I3- is often

labeled as the hole carrier to draw similarities with the conventional pn-junction

solar cells. Similarly, I- is produced at the counter-electrode and diffused to the

opposite direction in the electrolyte. This redox reaction in the electrolyte is a

two-electron reaction (Ferber et al. 1998):

3I- ↔ I3- + 2℮- (7)

Which is composed of series of successive reactions:

(I- ↔ I + ℮-)× 2 charge transfer reaction, (8)

2I ↔ I2 fast chemical reaction, (9)

39

I2 + I- ↔ I3- fast chemical reaction, (10)

Recombination of the generated electrons with holes in the dye-sensitized

nanostructured TiO2 electrode can happen either after the electron injection or

during its migration in the TiO2 electrode on its way to the electrical back contact.

(Zaban et al. 1997.)

In the dye-sensitized TiO2 electrode, there is on the contrary vast amount of

particle boundaries and a huge surface to volume ratio. However, the dye solar

cell has no recombination losses at the grain boundaries at all. The reason is

only electrons are transported through the semiconductor particles, while holes

(oxidized ions) are carried by the electrolyte. So, the dye cell works as a majority

carrier device, which is similar to a metal-semiconductor junction or a Schottky

diode. (Green 1982, p. 175.)

According to Huang et al. (1997) the net recombination reaction at the TiO2 -

electrolyte interface is a two electron reaction:

I3- + 2℮- → 3I- (11)

Composed of sub-reactions

I3- ↔ I2 + I- (12)

I2 + ℮- → I2- (13)

2I2-

I3

- + I- (14)

40

The reaction equation (13) shows that the actual electron acceptor in the

recombination reaction is I2. The last equation is a slow dismutation reaction and

rate limiting in the net recombination reaction.

41

7 PERFORMANCE AND APPLICATION OF TiO2 DYED SOLAR

CELL

7.1 Energy Conversion Efficiency

The high energy conversion efficiencies performed by the dye-sensitized solar

cells are one important reason for DSSC research. As we can see from table 7.1,

the Grätzel's group reported the highest efficiencies (Green 2001, Grätzel 2000,

Nazeeruddin et al. 1993, O'Regan & Grätzel 1991):

Table 7.1 Some of the best or otherwise interesting performance results from the

dyesensitized nanostructured TiO2 solar cells (laboratory scale) showing the

used dye, the cell efficiency, the reported area of the cell and the used

illumination in the efficiency measurement. (Halme 2002)

42

An efficient charge transfer sensitizer dye is a prerequisite for high efficiency in

the DSSCs. But at the same time, the dye developed by the Grätzel group is the

most advanced. Natural dyes extracted from berries have been demonstrated in

DSSCs with 0.56% cell efficiency (Cherepy et al. 1997).

Table 7.2 shows the purpose of replacing titanium dioxide with some other large

band-gap semiconductor, such as In2S3/In2O3 (Hara et al. 2000b), SnO2 (Fungo

et al. 2000), ZnO (Bedja et al. 1997, Rensmo et al. 1997, Hara et al. 2000a),

Nb2O5 (Sayama et al. 1998), CdS and CdSe (Hodes et al. 1992), but this

replacement is not very successful yet.

Table 7.2 Some results reported using other semiconductors than TiO2 in the

DSSC shoving the used dye, the cell efficiency, the reported area of the cell and

the used illumination in the efficiency measurement. (Halme 2002)

7.2 Long Term Stability

The stability of the dye-sensitized solar cells depends greatly on the designed

chemical composition and the materials of the cell. Those unwanted impurities

which are possibly included during the preparation of the cells affect stability of

dye-sensitized solar cell too.

43

Table 7.3 Degradation factors and their effects on the cell performance. (Halme

2002)

Stable means here roughly: "no significant permanent decrease in performance

observed". As we can see from Table 7.3, visible light soaking with no or little UV

does not affect the cell performance. UV light has a long term effect of the cell

performance, but adding efficient chemicals such as MgI2 can reduce this effect

significantly. On the other hand, thermal stability can be obtained by choosing

the electrolyte solvent correctly (e.g. purified propionitrile).

Certain chemicals can enhance the stability of the cells. Especially

4-tertbutylpyridine (TBP) was found to increase the tolerance towards water in

the electrolyte (Rijnberg et al. 1998), MgI2 and CaI2 increased the tolerance

towards UV light (Hinsch et al. 2001a). Pure nitrile based solvents such as

acetonitrile and propionitrile were the most stable solvents we have found now.

According to research and experience, the importance of high chemical purity of

the solvent and inert handling should be emphasized (Hinsch et al. 2001a).

44

8 SUMMARY

Coating improves the printing properties of paper and provides coated paper

and paperboard enhanced smoothness, better ink receptivity, more whiteness,

and more gloss.

Volume fraction of coating pigment is important. The smaller the particle size is,

the glossier will be the coating. Yet there are still other pigment properties that an

ideal pigment should have, titanium dioxide matches many of them.

As a commonly used coating pigment in paper industry, titanium dioxide

pigments, TiO2 pigments have very high refractive indices, whiteness, and high

reflectance in the visible region of light and optimal particle size. These

properties make them the most efficient white pigments. And titanium dioxide is

also the most stable pigment; this is one important reason why titanium dioxide

can be used in making dye solar cell.

Sustainable and renewable generation of energy/electricity by photovoltaic

systems is being developed; solar electricity energy technology is a hot spot

today. Solar cell producing electricity directly from solar radiation, is near the

ideal way to use natural renewable energy. In the future, grid connected PV

system power generation might be widely used, thus, the cost of solar electricity

needs to be reduced.

The application of titanium dioxide solar cell requires only a low power output.

Less stringent efficiency requirements of titanium dioxide solar cell make the

manufacturing process more flexible, thus reducing manufacturing costs. Lower

manufacturing costs and more flexibility in manufacturing process open the way

for alternative production processes.

45

Because of its high efficiency, low manufacturing cost and long term stability,

titanium dioxide is an ideal new material for solar electricity industry.

46

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